Production of stable aqueous dispersions of carbon nanotubes

ABSTRACT

Methods of producing stable dispersions of single-walled carbon nanotube structures in solutions are achieved utilizing dispersal agents. The dispersal agents are effective in substantially solubilizing and dispersing single-walled carbon nanotube structures in aqueous solutions by coating the structures and increasing the surface interaction between the structures and water. Exemplary agents suitable for dispersing nanotube structures in aqueous solutions include synthetic and natural detergents having high surfactant properties, deoxycholates, cyclodextrins, chaotropic salts and ion pairing agents. The dispersed nanotube structures may further be deposited on a suitable surface in isolated and individualized form to facilitate easy characterization and further processing of the structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional PatentApplication Serial No. 60/303,816, entitled “Isolation and Purificationof Single Walled Carbon Nanotube Structures”, and filed Jul. 10, 2001The disclosure of the above-mentioned provisional application isincorporated herein by reference in its entirety.

GOVERNMENT INTERESTS

[0002] This invention was made with Government support under contractNCC9-41 awarded by the National Aeronautics and Space Administration.The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] 1. Technical Field

[0004] The present invention relates to methods for isolating andpurifying single-walled carbon nanotubes from contaminating materials,such as carbon and metal catalyst particles, present in the unpurifiedmaterial following production of the single-walled carbon nanotubestructures. Specifically, the present invention relates to utilizingsolutions of suitable dispersal agents to isolate and purify individualsingle-walled carbon nanotube structures from a raw material includingbundles of nanotube structures.

[0005] 2. Description of the Related Art

[0006] There has been significant interest in the chemical and physicalproperties of carbon nanotube structures since their discovery in 1991,due to the vast number of potential uses of such structures,particularly in the field of nanotechnology, composite materials,electronics and biology. Accordingly, there has been an increase indemand in recent years for carbon nanotube structures for research andapplication purposes, resulting in a desire to produce in an efficientmanner single-walled carbon nanotube (SWCNT) structures that are free ofimpurities and easily separable for their proper characterization.

[0007] The three most common manufacturing methods developed for theproduction of SWCNT structures are high pressure carbon monoxide (HipCO)processes, pulsed laser vaporization (PLV) processes and arc discharge(ARC) processes. Each of these processes produce SWCNT structures bydepositing free carbon atoms onto a surface at high temperature and/orpressure in the presence of metal catalyst particles. The raw materialformed by these processes includes SWCNT structures formed as bundles oftubes embedded in a matrix of contaminating material composed ofamorphous carbon (i.e., graphene sheets of carbon atoms not formingSWCNT structures), metal catalyst particles, organic impurities andvarious fullerenes depending on the type of process utilized. Thebundles of nanotubes that are formed by these manufacturing methods areextremely difficult to separate.

[0008] In order to fully characterize the physical and chemicalproperties of the SWCNT structures formed (e.g., nanotube length,chemical modification and surface adhesion), the contaminating matrixsurrounding each structure must be removed and the bundles of tubesseparated and dispersed such that each SWCNT structure may beindividually analyzed. By maintaining an appropriate dispersal ofindividual SWCNT structures, characterization of the nanotubes formedmay be accomplished in a mechanistic manner. For example, it isdesirable to easily analyze and characterize dispersed SWCNT structures(e.g., determine change in nanotube length, tensile strength orincorporation of defined atoms into the carbon matrix of the SWCNTstructure) based upon a modification to one or more elements of amanufacturing method.

[0009] It is further highly desirable to produce individual and discreteSWCNT structures in a form rendering the structures easily manipulablefor use in the previously noted fields. At best, existing methodologiescapable of physically manipulating discrete material components requireelements that are measured on micron-level dimensions rather than thenanometer level dimensions of conventional partially dispersed andpurified SWCNT structures. However, biological systems routinelymanipulate with precise spatial orientation discrete elements (e.g.,proteins) having physical dimensions on the order less than SWCNTstructures. Thus, if SWCNT structures could be biologically derived sothat biological “tools”, such as immunoglobulins or epitope-specificbinding proteins, could be utilized to specifically recognize andphysically manipulate the structures, the possibility of accuratelyspatially orienting of SWCNT structures becomes feasible. In order forthis approach to be realized, the SWCNT structures must be individuallyseparated from the raw material in a manner consistent with the optimalfunctioning of biological compounds during both the biological SWCNTderivitization and the manipulation processes. In other words, the SWCNTstructures must be produced as individual, freely dispersed structuresin an aqueous buffer system that exhibits a nearly neutral pH at ambienttemperatures in order to effectively manipulate the structures.

[0010] Current methods for purifying and isolating SWCNT structures byremoving the contaminating matrix surrounding the tubes employ a varietyof physical and chemical treatments. These treatments include hightemperature acid reflux of raw material in an attempt to chemicallydegrade contaminating metal catalyst particles and amorphous carbon, theuse of magnetic separation techniques to remove metal particles, the useof differential centrifugation for separating the SWCNT structures fromthe contaminating material, the use of physical sizing meshes (i.e.,size exclusion columns) to remove contaminating material from the SWCNTstructures and physical disruption of the raw material utilizingsonication. Additionally, techniques have been developed to partiallydisperse SWCNT structures in organic solvents in an attempt to purifyand isolate the structures.

[0011] All of the currently available methods are limited for a numberof reasons. Initially, it is noted that current purification methodsprovide a poor yield of purified SWCNT structures from raw material. Afinal SWCNT product obtained from any of these methods will alsotypically contain significant amounts of contaminating matrix material,with the purified SWCNT structures obtained existing as ropes or bundlesof nanotubes thereby making it difficult to analyze and characterize thefinal SWCNT structures that are obtained. These methods furthertypically yield purified SWCNT structures of relatively short lengths(e.g., 150-250 nm) due to the prolonged chemical or physical processingrequired which causes damage to the nanotubes. Additionally, a number ofisolation techniques currently utilized require organic solvents orother noxious compounds which create environmental conditions unsuitablefor biological derivitization of SWCNT structures. Organic solventscurrently utilized are capable of solubilizing SWCNT structures inbundles and not individual, discrete tubes. Furthermore, presentisolation techniques require prolonged periods of ultra-speedcentrifugation (i.e., above 100,000×g) in order to harvest nanotubestructures from solvents or other noxious compounds used to removecontaminating matrix material from the nanotubes.

[0012] Presently, the overwhelming problem for industrial and academiclaboratories engaged in the use of carbon nanotubes for research as wellas other applications is the limited source of discrete, completelyseparated SWCNT structures. Investigations into the vast potential ofuses for SWCNT structures are being hampered by the limited supply ofwell characterized SWCNT material free of significant amounts ofcontaminants like amorphous carbon and metal catalyst particles.

[0013] Accordingly, there presently exists a need for harvesting highyields of purified SWCNT structures from the raw material of a carbonnanotube production process in a fast and efficient manner to meet thedemand for such structures. Additionally, it is desirable to provideSWCNT structures as discrete and individual structures (i.e., notbundled together), having suitable lengths and well characterized forbiological derivitization and easy manipulation.

SUMMARY OF THE INVENTION

[0014] Therefore, in light of the above, and for other reasons that willbecome apparent when the invention is fully described, an object of thepresent invention is to provide a rapid and effective method ofisolating and purifying SWCNT structures disposed within a raw materialcontaining contaminants to obtain a high product yield of quality SWCNTstructures having appropriate lengths suitable for differentapplications.

[0015] Another object of the present invention is to provide a method ofdispersing isolated and purified SWCNT structures in solution from theraw material so as to yield discrete and separated nanotube structuressuitable for different applications.

[0016] A further object of the present invention is to provide a methodof dispersing isolated and purified SWCNT structures in a suitablesolution to render the structures suitable for biological derivitizationprocedures to effect easy manipulation of the SWCNT structures.

[0017] A further object of the present invention is to produce anaqueous dispersion of single, discrete SWCNT's that remains stable overa prolonged period of time (i.e. weeks to months), a dispersion in whichaggregation or “flocking” of SWCNT's does not occur.

[0018] The aforesaid objects are achieved in the present invention,alone and in combination, by providing a method of dispersing a matrixof raw material including SWCNT structures and contaminants in anaqueous solution containing a suitable dispersal agent to separate theindividual SWCNT structures from the matrix, thus purifying anddispersing the structures within the solution. In solution, thedispersal agent surrounds and coats the individual SWCNT structures,allowing the structures to maintain their separation rather thanbundling together upon separation of the structures from solution.Suitable dispersal agents useful in practicing the present invention aretypically reagents exhibiting the ability to interact with hydrophobiccompounds while conferring water solubility. Exemplary dispersal agentsthat can be used in the present invention include synthetic and naturaldetergents, deoxycholates, cyclodextrins, poloxamers, sapogeninglycosides, chaotropic salts and ion pairing agents.

[0019] The above and still further objects, features and advantages ofthe present invention will become apparent upon consideration of thefollowing detailed description of specific embodiments thereof,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1a is plot of % transmission (% T) values for aqueoussolutions containing three synthetic detergents having varyingsurfactant strengths.

[0021]FIG. 1b is a plot of % transmission (% T) vs. time for the aqueoussolutions of FIG. 2a, wherein the solutions have undergone evaporation.

[0022]FIG. 1c is a plot of % transmission (% T) vs. time for aqueoussolutions of FIG. 2a, wherein the solutions have undergone noevaporation.

[0023]FIG. 2 is a plot of % transmission (% T) vs. time for aqueoussolutions containing taurocholic acid, Poloxamer 188, saponin andmethyl-β-cyclodextrin.

[0024]FIG. 3 is a plot of % Transmission values vs. fraction #'smeasured during fractionation of methyl-β-cyclodextrin dispersed SWCNTstructures in a 5000 MW size exclusion column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention relates to a method for purifying andisolating SWCNT structures from raw material by dispersing thestructures in an aqueous solution with a biologically active dispersalagent. The biologically active dispersal agent effects a separation ofthe SWCNT structures from contaminating material such that the purifiedSWCNT structures exist as a dispersion of individual and discrete SWCNTstructures in solution. As used herein, the term “raw material” refersto material formed by any process for producing single-walled carbonnanotubes, including, without limitation, the three processes describedabove. The raw material typically contains SWCNT structures embedded ina matrix of contaminating material. The terms “contaminating material”and “contaminants”, as used herein, refer to any impurities or othernon-SWCNT components in the raw material including, without limitation,amorphous carbon and metal catalyst particles.

[0026] As previously noted, the current methods employed for purifyingand harvesting SWCNT structures have met with limited success due inpart to the traditional view of SWCNT structures as chemical compounds.In a departure from the traditional view, SWCNT structures areconsidered here as being similar to biologically derived structures.Some noted properties of SWCNT structures are as follows (not all ofwhich must be present): they are typically insoluble in water; theytypically self associate as bundles or ropes; they are made exclusivelyof carbon; and each end of a carbon nanotube will typically exhibitdifferent physiochemical properties. The physical properties of carbonnanotubes are in fact very similar to lipids, which are a class ofbiological compounds insoluble in water but capable of being solubilizedin aqueous solutions including suitable lipid dispersing reagents. Assuch, the inventors recognized that SWCNT structures are readilydispersable within an aqueous solution containing a reagent typicallysuitable for dispersing proteins or lipids in aqueous solutions.

[0027] Reagents considered effective in suitably dispersing SWCNTstructures in aqueous solution are referred to as dispersal agents. Adispersal agent can be any suitable reagent that is effective insubstantially solubilizing and dispersing SWCNT structures in an aqueoussolution by increasing the interaction at the surface interface betweeneach nanotube structure and water molecules in solution. The underlyingmechanisms whereby a dispersal agent brings about dispersion ofindividual SWCNT structures, from the “bundles” or “ropes” in which theyare constitutively formed, into an aqueous solution is primarily basedupon the ability of the dispersal agent to break down the molecularforces at the surface of the SWCNT preventing water molecules frominteracting with the SWCNT surface. In addition to this property, due tothe large surface area of the SWCNT, it also essential that thedispersal agent have a molecular structure that maximizes its ability toreduce hydrophobic interactions between individual SWCNT's, while alsobeing of a small enough size to easily penetrate into the inter-SWCNTspaces. A further requirement of an efficient SWCNT dispersal agent isthat it also can remain in aqueous solution at a high enoughconcentration so that a useful dispersal agent concentration ismaintained for SWCNT “bundle” or “rope” dispersal, even after a portionof the original amount in solution has been utilized for the dispersionof non-SWCNT contaminants in the raw nanotube material. The dispersalagent is typically added to an aqueous solution in an effective amountto substantially purify and disperse SWCNT structures in solution. Theeffective amount of dispersal agent will vary based upon the type ofdispersal agent utilized in a particular application.

[0028] The dispersal agents are typically synthetic or naturallyoccurring detergents or any other composition capable of encapsulatingand suitably solubilizing hydrophobic compounds in aqueous solutions.Exemplary dispersal agents include, without limitation, synthetic ornaturally occurring detergents having high surfactant activities such asNonidet P-40 (NP-40), polyoxyethylene sorbitol esters (e.g., TWEEN® andEMASOL™ series detergents), poloxamers (e.g., Poloxamer 188 and thePluronic™ series of detergents) and ammonium bromides and chlorides(e.g., cetyltrimethylammonium bromide, tetradecylammonium bromide anddodecylpyrimidinium chloride), naturally occurring emulsifying agentssuch as deoxycholates and deoxycholate-type detergents (e.g.,taurocholic acid), sapogenin glycosides (e.g., saponin) andcyclodextrins (e.g., α-, β- or γ-cyclodextrin), chaotropic salts such asurea and guanidine, and ion pairing agents such as sulfonic acids(e.g.,1-heptane-sulfonic acid and 1-octane-sulfonic acid).

[0029] Naturally occurring emulsifying agents such as taurocholic acidand cyclodextrins are highly effective in solubilizing and dispersingSWCNT structures and in facilitating biological derivitization of thepurified and isolated SWCNT structures. In particular, cyclodextrinshave a three dimensional doughnut shaped orientation with a “torsional”structure composed of glucopyranose units. The “torsional” structure ofa cyclodextrin molecule allows it to attract and interact with thesurface of a SWCNT structure within its central hydrophobic region, evenwhen physically altered from a round “doughnut” shape to a twisted“doughnut” shape, while maintaining an outer hydrophilic surfacerendering the molecule soluble in aqueous solutions. The solubility ofnative cyclodextrins in water may also be increased nearly tenfold bysubstitution of, e.g., methyl or hydroxypropyl groups on thecyclodextrin molecule. Greater solubility of the cyclodextrin in watertranslates to a greater dispersion and isolation of individual SWCNTstructures in solution. Two exemplary cyclodextrin derivatives that arehighly effective in dispersing SWCNT structures in solution aremethyl-β-cyclodextrin (MβC) and 2-hydroxypropyl-β-cyclodextrin(2-HP-β-C). However, it is noted that any cyclodextrin (i.e., α, β orγ), or any suitable derivative thereof, may be utilized in accordancewith the present invention. Further, cyclodextrins are useful forbiological derivitization of SWCNT structures which have been isolatedin solution. Taurocholic acid (TA), which is exemplary of a suitabledeoxycholate-type detergent capable of substantially dispersing SWCNTstructures in solution, is produced naturally in mammalian liver tissue.It is also highly effective in facilitating biological derivitization ofpurified and isolated SWCNT structures because, like the cyclodextrins,TA has a molecular shape that allows a large surface area of SWCNTstructures to be coated per molecule of TA. Typically, cyclodextrins anddeoxycholates may be utilized to suitably disperse SWCNT structuresaccording to the present invention in concentrations ranging from about5 mg/ml to about 500 mg/ml of aqueous solution, with a preferableconcentration of about 50 mg/ml.

[0030] Sapogenin glycosides (e.g. saponin), another naturally occurringclass of emulsifying agent of plant origin, are also capable ofdispersing SWCNT structures. Like both the cyclodextrins anddeoxycholate type detergents, these compounds are amphiphilic in nature,have a high solubility in water and can act as protective colloids tonormally water insoluble hydrophobic compounds (i.e., a SWCNT structure)in an aqueous solution. Solubilization of raw SWCNT material has beenachieved at concentrations between 0.1 mg/ml up to 50 mg/ml of aqueoussolution in the present invention, with a preferable concentration ofabout 10 mg/ml.

[0031] Synthetic detergents suitable for use as dispersal agents herewill typically have a high surfactant activity and be utilized inamounts of about 50-95% of their critical micelle concentration (CMC)values. These high surfactant detergents are capable of overcominghydrophobic forces at the SWCNT surface/aqueous solution interface bycoating the SWCNT structures to establish suitable solubility of theSWCNT structures in solution. The surfactant properties of a syntheticdetergent may be characterized in terms of a hydrophilic-lipophilicbalance (HLB), which provides a measurement of the amount of hydrophilicgroups to hydrophobic groups present in a detergent molecule. Inparticular, synthetic detergents that are suitable for use as dispersalagents here have an HLB value between about 7 and about 13.2. Limitingthe concentration of the synthetic detergent to a suitable level belowits CMC will also ensure adequate dispersion of the SWCNT structureswithout the formation of floccular material due to self-association ofthe detergent molecules. Additionally, chaotropic salts (e.g., urea andguanidine) are typically utilized as dispersal agents in concentrationsranging from about 6M to about 9M in solution (wherein “M” refers tomolarity), whereas ion pairing agents are typically utilized asdispersal agents in concentrations ranging from about 1 mM to about 100mM in solution.

[0032] While selection of a suitable dispersal agent as well as asuitable concentration is important for achieving a desirable dispersionof SWCNT structures in aqueous solution, other factors may also enhancethe dispersing effect of the dispersal agent. Exemplary factors thataffect dispersion of SWCNT structures in aqueous solutions include,without limitation, the pH of the solution, cation concentration (e.g.,sodium, potassium and magnesium) in solution, and other conditions suchas operating temperature and pressure. Indeed, due to the uniqueproperties of the solvent in this case, namely water, specifically theunique chemical interactions that can exist between individual watermolecules (i.e. hydrogen bonding, molecular aggregation) it is predictedthat decreasing the operating temperature (rather than increasing thetemperature as is the case in most common chemical reactions) willincrease the ability of a dispersal agent to disperse SWCNT material dueto a reduction in hydrophobic interactions between the SWCNT surface andthe water molecules at lower operating temperatures (i.e. between 0° C.and 10° C.). By interacting with the surface of individual SWCNTstructures, the dispersal agent molecules typically surround and coatthe exposed hydrophobic surface of the SWCNT. This interaction resultsin the separation of individual SWCNT structures from the bundles inwhich they were formed and from contaminating matrix material, and dueto the amphiphilic nature of the dispersal agent maintains the nowdiscretely separated, individual SWCNT structures in the form of anaqueous dispersion or colloidal solution.

[0033] The raw material containing SWCNT structures typically is addedto an aqueous solution containing the dispersal agent and appropriatelymixed (e.g., by mechanical agitation or blending) to ensure adequateinteraction and coating of dispersal agent molecules with SWCNTstructures. While the amount of SWCNT material that may be added to anaqueous dispersal agent solution to obtain an effective dispersion ofSWCNT structures typically depends upon factors such as the specificdispersal agent utilized and its concentration in solution, effectivedispersions have been achieved utilizing concentrations as high as 1mg/ml of SWCNT structures in aqueous dispersal agent solution. Uponadequate mixing, the solution containing the dispersed SWCNT structuresmay be filtered with an appropriately sized filter (e.g., about 0.05-0.2μm filtration) to remove any insoluble material (e.g., matrixcontaminants) remaining in solution. Typically, a 0.2 μm filter isutilized to ensure adequate removal of contaminants while preventingcaking of the filter and loss of dispersed SWCNT structures. However,smaller pore size filters may also be utilized to ensure more efficientremoval of contaminants. In situations where a smaller pore size filteris implemented, any SWCNT structures that may have become trapped in thefilter cake may be recovered by resuspension of the cake in dispersalagent solution and repeating filtration steps as necessary to obtain adesirable yield. Alternatively, a standard cross-flow filtration systemcan be utilized to reduce the amount of caking that occurs on thesurface of the filter.

[0034] Additional processing steps, such as centrifugation or otherseparation techniques, may also be utilized to remove insoluble materialand excess dispersal agent from solution after the SWCNT structures havebeen suitably dispersed therein. Specifically, the SWCNT structures maybe washed to remove excess dispersal agent by subjecting the solution tocentrifugation at speeds ranging from about 100×g to about 10,000×g tosediment SWCNT structures. The SWCNT structures may then be removed fromsolution and re-dispersed in distilled water. The washing process may berepeated any desired number of times to ensure adequate removal ofexcess dispersal agent. The SWCNT structures may also be separated fromexcess dispersal agent and other contaminants in solution via dialysisor the use of an appropriate size exclusion column (e.g., a 5000 MW sizeexclusion column). The resultant solution, which contains substantiallyisolated and purified SWCNT structures coated with dispersal agent, ishighly useful in a variety of applications, particularly nanotechnologyresearch. As for example in the case of protein biochemistry, whereprotein function may be negatively impacted by the use of a particulardispersal agent as a prerequisite to enable the extraction of theprotein from contaminating matrix, once dispersal of the protein into anaqueous solution has been achieved, the offending dispersal agent can besubstituted with a second dispersal agent. This second dispersal agentalso maintains the protein molecule in aqueous solution but is moreappropriate for those applications where protein function is paramount.In a similar fashion, once dispersal of SWCNT material has beenachieved, if necessary the primary dispersal agent can be substitutedwith a second dispersal agent that is more suitable for the envisioneduse. For example, a cyclodextrin can be used as the primary dispersalagent to produce a stable dispersion of individual SWCNT's in an aqueoussolution. The cyclodextrin dispersal agent can then be substituted with,for example, a secondary dispersal agent such as Poloxamer 188. Thesecondary dispersal agent provides a matrix material that coats thesurface of individual SWCNT's that is easily polymerized to form, forexample, an SWCNT-containing composite material, or a readily accessiblesource of chemical groups now associated with the surface of thedispersed SWCNT's that can be easily modified using standardchemistries.

[0035] One central aim of the present invention is to ensure thatSWCNT's are dispersed in an aqueous solution in the form of individual,discrete nanotubes (i.e., a complete dispersion). One characteristic ofSWCNT material dispersed in an aqueous solution that contains SWCNT's,that remain either in non-dispersed bundles or that have beenindividually separated but have re-associated into bundles (i.e., anincomplete dispersion), is that the SWCNT material re-associates intolarge aggregates or “flocs” which in turn sediment out of solution. Thisprocess occurs within a matter of minutes to hours, even after filteringthe dispersion through a 0.2 micron filter. In the case of a completedispersion, filtering the solution through a 0.2 micron filter resultsin a colored liquid that essentially does not flock or aggregate overtime and remains stable (i.e., no flocking) for extended periods oftime. As long as the relative concentration of the dispersed SWCNT's ordispersal agent in the aqueous solution is not increased by evaporationof water the dispersion remains stable (i.e., SWCNT's remain as singlediscrete nanotubes in the aqueous solution).

[0036] The maximum concentration of SWCNT material that can bemaintained in solution by a particular dispersal agent is related to thetotal surface area of the individual SWCNT's present in the solution. Acomplete aqueous dispersion of SWCNT material exists where there is abalance between the amount of SWCNT surface area exposed to the solvent(i.e., water) and the amount of dispersal agent available to interactwith that exposed SWCNT surface in order to confer water solubility onthe SWCNT. The amount of dispersal agent available to interact with theexposed SWCNT surface in aqueous solution is in turn dependent on thewater solubility of the particular dispersal agent and the amount ofSWCNT surface area that each individual dispersal agent molecule iscapable of interacting with. As such, there is a maximal amount of SWCNTmaterial that can be completely dispersed at a particular concentrationof a particular dispersal agent. The maximal amount of SWCNT materialthat can be dispersed using a particular dispersal agent is achieved ator below the concentration of dispersal agent in aqueous solution whereself-association of the dispersal agent occurs (known as the CMC in thecase of detergents and Cloud Point in the case of emulsifying agents).In addition, the amount of SWCNT material that can be dispersed in sucha dispersal agent solution cannot however exceed the saturationconcentration of individual, discrete SWCNT's in solution, which due totheir large physical size exhibit colloidal properties. This maximumSWCNT concentration is in turn dependent on the length of the SWCNT(i.e., its molecular size), the longer the SWCNT the lower the maximalconcentration that can be maintained as a complete dispersion in anaqueous solution at a constant dispersal agent concentration. Based onthis understanding, for a particular dispersal agent dissolved at itsoptimal concentration in water, there is also a maximum concentration(i.e., number) of SWCNT's that can be maintained as a complete aqueousdispersion, where that maximum number is directly related to the surfacearea of the SWCNT's in solution.

[0037] For example, a SWCNT of 100 nm in length and 1 nm in diameter hasan exposed external surface area of 100π nm². A SWCNT of 10,000 nm (e.g.10 microns) in length and 1 nm in diameter has an exposed externalsurface area of 10,000π nm². As such, it requires the same amount ofdispersal agent to maintain one hundred, 100 nm long SWCNT's as acomplete dispersion as it does to keep a single 10 micron SWCNT incomplete dispersion. This example demonstrates the importance of therelationship between (1) molecular shape of the dispersal agent (i.e.,the amount of SWCNT surface area that a single molecule of dispersalagent can interact with), (2) concentration of the dispersal agent insolution, (3) overall exposed SWCNT surface area (related to SWCNTlength) and (4) the maximal amount of SWCNT material that can exist as acomplete aqueous dispersion.

[0038] The following examples disclose specific methods for isolatingand purifying SWCNT structures from raw material containingcontaminants. Specifically, NP-40, TA, Poloxamer 188, saponin and acyclodextrin derivative are utilized to show the effect of each indispersing SWCNT structures in aqueous solution. The raw materialcontaining SWCNT structures for each example was obtained utilizing aPLV process. However, it is noted that the SWCNT structures may beisolated and purified utilizing raw material provided via any process.It is further noted that the examples are for illustrative purposes onlyand in no way limit the methods and range of dispersal agentscontemplated by the present invention.

EXAMPLE 1

[0039] Raw material containing bundled SWCNT structures was mixed intothree synthetic detergent solutions known for solubilizing proteins andlipids in aqueous solutions. The three synthetic detergents utilizedwere NP-40, SDS and TX-100. These detergents were selected due to theirdiffering physical properties and to demonstrate how the surfactantactivity of the detergent affects the dispersion of SWCNT structures insolution. SDS is a strong anionic detergent that solubilizes compoundsin water by virtue of coating the compounds with a layer ofnegatively-charged, water soluble detergent molecules. In contrast, bothTX-100 and NP-40 are non-ionic detergents that function via hydrophobicinteractions with the surface of a compound, thereby forming a watersoluble layer of detergent molecules around the water insolublecompound. The surfactant properties (i.e. ability to decrease surfacetension between aqueous and non-aqueous phases) for NP-40 are muchgreater than SDS and TX-100. Reported HLB values for each of thesedetergents are as follows (e.g., see Kagawa, Biochim. Biophys. Acta 265:297-338 (1972) and Helenius et al., Biochim. Biophys. Acta 415: 29-79(1975)): Detergent HLB SDS 40 TX-100 13.5 NP-40 13.1

[0040] Three aqueous solutions were each prepared as follows. A 1 mg(total dry weight) amount of raw material was solubilized in 1 ml ofdouble glass-distilled, deionized water (ddH₂O) containing one of thedetergents (e.g.., SDS, TX-100 or NP-40) at 50% of its respective CMCvalue. Each solution was subsequently vortexed for 30 minutes at roomtemperature. The resultant dispersions were passed through a 0.2 μmcellulose acetate filter to remove any particulate matter. Conventionalspectroscopy methods were employed to measure the percent transmission(% T) of each solution at a wavelength of 450 nm (path length of 3 mm).

[0041] The % T value of each the solutions was measured to provide anindication of solution color and thus comparatively determine theability of each detergent to effectively disperse SWCNT structureswithin solution. Specifically, % T values are inversely proportional tothe degree of color in solution. If SWCNT structures are bundledtogether in a particular solution (or begin to re-aggregate intobundles), flocular material forms removing SWCNT structures fromsolution by sedimentation, thus decreasing the color and increasing the% T value over time. Alternatively, if SWCNT structures remain dispersedin solution, no flocking occurs and the color solution remainsconsistent. Therefore, a lower % T value measured in the filtrate wouldindicate a higher level of dispersion of SWCNT material in solution, anda constant % T over time reflects a stable SWCNT dispersion.

[0042] The plots illustrated in FIGS. 1a-1 c provide % T data forsolutions containing SDS, TX-100 and NP-40, respectively, with andwithout SWCNT structures. The unshaded bar portions in FIG. 1a represent% T values measured for each detergent solution absent any raw material.The % T value for the shaded bar portions represent % T values measuredfor each detergent solution containing SWCNT structures at a timeshortly after 0.2 μm filtration of the solution. The shaded bar data ofFIG. 2a clearly indicates that NP-40, which has the greatest surfactantproperties, has a much lower % T value than both SDS and TX-100 and thusprovides a substantially more effective dispersion of SWCNT structuresin aqueous solution.

[0043] To illustrate the effect of detergent concentration on SWCNTdispersion in solution, the solutions containing SWCNT structures wereallowed to evaporate from an initial volume of 150 μl to a final volumeof 50 μl over a period of 16 hours at room temperature. Intermittent % Tmeasurements were taken, and the results are illustrated in FIG. 1b. The% T values for each solution containing a detergent and SWCNT structuresincreased with time (i.e., correlating with a decrease in color), whichcoincided with a noticeable appearance of flocular material in thedetergent dispersions thus indicating that nanotubes were beginning tore-associate into larger bundles that were insoluble in water. The testresults indicate that, as the detergent concentration increases aboveits CMC value, micelle formations occur in solution resulting in reduceddispersion of the SWCNT structures. Thus, selection of detergentconcentration is very important in maintaining dispersion of the SWCNTstructures in solution. Alternatively, the results (FIG. 1b) couldindicate that as the volume of the solution decreased due to waterevaporation, not only did the relative concentration of the detergentincrease above its CMC resulting in a functional decrease in the amountof dispersal agent available to maintain discrete individual SWCNT's insolution, but so too did the relative concentration of the SWCNT's inthe solution. Due to the colloidal nature of the SWCNT dispersion, thisprocess could in isolation, or, in conjunction with the detergentconcentration rising above the CMC, result in re-aggregation or“flocking” of SWCNT's in the dispersion, an event that in turn isreflected by an increase in % T.

[0044] A further test was conducted with solutions prepared in asubstantially similar manner as the previous solutions. However, thesesolutions were stored in sealed vials at room temperature so as toprevent their evaporation. As indicated by the data depicted in FIG. 1c,there was relatively no change in % T value for each of the differentdetergent solutions and no noticeable appearance of flocular materialafter a 72 hour period, or an increase in % T after a second round offiltration through a 0.2 μm filler.

[0045] The data of example 1 indicates that a strong surfactant such asNP-40 is highly effective in dispersing SWCNT structures in aqueoussolutions when utilized in an effective amount. Further, NP-40 canmaintain a suitable dispersion of the structures in solution forextended periods of time. Weaker surfactants having HLB values greaterthan 13.2, such as SDS and TX-100, may provide some dispersion but willnot be effective in substantially isolating and purifying SWCNTstructures from raw material.

EXAMPLE 2

[0046] Aqueous solutions of each of the TA, Poloxamer 188, saponin andMβC were prepared alone and with raw material as follows. Specifically,each solution was prepared by solubilizing 1 mg of the raw material in 1ml of ddH₂O containing either 50 mg/ml of TA, 50 mg/ml of MβC, 10 mg/mlof saponin or 2% (v/v) Poloxamer 188. Each resultant solution wasvortexed for 30 minutes at room temperature and then filtered through a0.2 μm cellulose acetate filter. The % T values were measured for thefiltrates at room temperature immediately after filtration, 72 hr afterstorage in a sealed vial of the original filtered solutions and againimmediately after a second filtration of the stored solutions. Dispersalagent/SWCNT-containing solutions were compared to aqueous solutionscontaining dispersal agent alone treated in an identical fashion.

[0047] The % T values illustrated in FIG. 2 reveals that the SWCNTstructures remained dispersed in the TA, Poloxamer 188, saponin and MβCfiltrates for the entire 72 hour period, as is evident from therelatively constant % T values measured for each filtrate over that timeperiod. As demonstrated in FIG. 1b, if flocking or re-aggregation ofdispersed SWCNT material occurs, this results in a decrease in the % Tof the dispersion. In addition, if flocking or re-aggregation of SWCNTmaterial in the dispersion (FIG. 2) had occurred to any extent duringthe 72 hour period of storage without water evaporation, filtration ofthis dispersion through a 0.2 μm filter for a second time will result inthe removal of this re-aggregated or flocked material resulting in adecrease in % T value of the dispersion. The data further indicates MβCfiltrates had considerably lower % T values, correlating to a greaterdispersion of SWCNT structures, than either the TA, the Poloxamer 188,or saponin filtrates and the NP-40 filtrate of FIG. 2c. No significantincreases in % T values were observed even after a second round of 0.2μm filtration of each filtrate after the 72 hour period. The resultsprovided in FIG. 2 clearly indicate that TA, saponin, Poloxamer 188 andMβC serve as highly effective dispersal agents, providing substantialdispersion of the SWCNT structures in aqueous solution for extendedperiods of time.

EXAMPLE 3

[0048] SWCNT structures dispersed in the TA and MβC solutions of theprevious example were separated from the impurities in solution bycentrifugation. Specifically, SWCNT structures sedimented out of a 1 mlvolume of either solution having a liquid column height of 2.5 cm at acentrifugation speed of 10,000×g. In addition, sub-populations ofSWCNT's dispersed in either NP-40, TA, MβC, saponin or Poloxamer 188 canbe collected from the same sample by using sequentially increasingcentrifugation speeds (e.g. 1,000×g, 2,500×g, 5,000×g and 7,500×g). Assuch, these results suggest that as is the case in biologicalseparations where differential centrifugation can be used to separatecellular structures based upon their size (e.g., see Techniques Reviewedin Subcellular Fractionation—A practical approach; edited by J. M..Graham and D. Rickwood, IRL Press, Oxford, 1996), a similar approach canbe utilized to collect SWCNT's of different sizes from the aqueousdispersions described here. It is noted that prior SWCNT purificationtechniques typically require centrifuigation speeds in excess of100,000×g to yield any sedimentation of SWCNT structures, anexperimental observation that is consistent with the presence of veryshort (i.e., less than about 250 nm) SWCNT's being present in suchprevious solutions. The ability to sediment SWCNT's from the aqueousdispersions described in the present invention at the relatively lowg-forces indicated above (i.e., below 10,000×g) indicates that the SWCNTstructures present in those dispersions must be much larger (i.e.,longer as the diameter of a SWCNT is a constant dimension ofapproximately 1 nm) than those previously produced and that requireabove 10,000×g forces to bring about sedimentation.

EXAMPLE 4

[0049] Aqueous MβC solutions containing dispersed SWCNT structures wereprepared as follows. Two hundred μg of SWCNT containing raw material wassolubilized in a 1 ml solution of ddH₂O containing 50 mg/ml of MβC. Thesolution was physically homogenized in a miniaturized inversion blenderat about 23,000 RPM. The resultant dispersion was subsequently vortexedfor 30 minutes at room temperature followed by 100×g centrifugation for10 minutes to sediment any remaining insoluble material. The resultantsupernatant was then passed through a 5000 MW cut-off gravity-fed sizeexclusion column (10 ml bed volume) in the following manner. One ml ofthe dispersed solution was placed on the top of the column, which hadbeen conditioned with 50 ml of ddH₂O. One ml fractions were thencollected from the base of the column as ddH₂O was added to the top ofthe column. The % T values were measured for each collected fraction. Aplot of the % T values vs. fraction# is illustrated in FIG. 3. Coloredfractions, as indicated by the decreasing % T values, were indicative ofdispersions in solution. Those colored fractions (i.e., fraction#'s 1-10of FIG. 5) were collected and pooled together. This procedure wasconducted to remove excess MβC from the SWCNT dispersions. The resultantsolution containing the dispersed SWCNT structures was centrifuged at10,000×g to sediment SWCNT structures from solution. The supernatant wasthen discarded and the sedimented SWCNT's were resuspended in distilledwater to form a stable aqueous dispersion of discrete separated SWCNT'scontaining little or no excess of dispersal agent. Again, an approachthat has commonly been used in biological science to achieve separationof biological molecules, namely size exclusion chromatography, can besuccessfully applied to the experimental problems encountered in theseparation and purification of SWCNT material. Based upon anunderstanding of biological separation techniques, the elution ofdifferent amounts of dispersed SWCNT's from a size exclusion columnafter different retention times (as indicated by differing % T values ineach of the fractions, i.e., Fraction #1-10, in FIG. 3) suggests thatdiscrete SWCNT's truly dispersed in an aqueous solution can be separatedand purified on the basis of their length using size exclusionchromatography in a similar fashion to that employed for separating andpurifying proteins of different sizes. In conjunction with the previousExamples 1-3, these data strongly suggest that the approach described inthe present invention to solving the experimental problems encounteredin the separation and purification of SWCNT material, based upon thenovel and innovative concept that the SWCNT structures behaveessentially as “biological” compounds, rather than as a product ofphysical or organic chemistry, has been successful.

[0050] Having described novel methods of producing stable aqueousdispersions of SWCNT's and corresponding products thereof, it isbelieved that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims.

What is claimed:
 1. A method of isolating single walled carbon nanotubestructures embedded within raw material, the method comprising: mixingsaid structures in a solution including an effective amount of adispersal agent to substantially disperse said structures within saidsolution.
 2. The method of claim 1, wherein mixing said structures insaid solution substantially separates said structures from contaminantsin said raw material.
 3. The method of claim 1, wherein said dispersalagent is selected from the group consisting of detergents with highsurfactant activities, deoxycholates, cyclodetrins, chaotropic salts,poloxamers, saponin glycosides and ion pairing agents.
 4. The method ofclaim 1, wherein said dispersal agent comprises a detergent, and saideffective amount of said detergent in said solution is no greater thanabout 95% of a critical micelle concentration of said detergent.
 5. Themethod of claim 1, wherein said effective amount of said detergent insaid solution is at least about 50% of a critical micelle concentrationof said detergent.
 6. The method of claim 1, wherein said dispersalagent comprises a detergent having a hydrophilic-lipophilic balancevalue no greater than about 13.2.
 7. The method of claim 1, wherein saiddispersal agent is a detergent selected from the group consisting ofNonidet P-40, Poloxamer 188, polyoxyethylene sorbitol esters, ammoniumbromides and ammonium chlorides.
 8. The method of claim 1, wherein saiddispersal agent is selected from the group consisting of cyclodextrins,saponin and taurocholic acid.
 9. The method of claim 8, wherein saideffective amount of said dispersal agent is no greater than about 500mg/ml.
 10. The method of claim 8, wherein said effective amount of saiddispersal agent is at least about 5 mg/ml.
 11. The method of claim 1,wherein said dispersal agent is a cyclodextrin derivative selected fromthe group consisting of methyl-β-cyclodextrin and2-hydroxypropyl-β-cyclodextrin.
 12. The method of claim 1, wherein saiddispersal agent is a chaotropic salt selected from the group consistingof urea and guanidine.
 13. The method of claim 12, wherein the effectiveamount of said chaotropic salt in said solution is in no greater thanabout 9M.
 14. The method of claim 12, wherein the effective amount ofsaid chaotropic salt in said solution is at least about 6M.
 15. Themethod of claim 1, wherein said dispersal agent is a sulfonic acid, andsaid effective amount of said sulfonic acid in said solution is nogreater than about 100 mM.
 16. The method of claim 15, wherein saidsulfonic acid is selected from the group consisting of1-heptane-sulfonic acid and 1-octane-sulfonic acid.
 17. The method ofclaim 1, further comprising: separating said structures from said rawmaterial in said solution by at least one of: passing said solutionthrough a filter to form a purified filtrate of said structures; andpassing said solution through a size exclusion column to form a purifiedsolution of said structures.
 18. The method of claim 17, wherein saidfilter includes a pore size no greater than about 0.20 μm.
 19. Themethod of claim 1, further comprising: centrifuging said solution at aspeed in a range no greater than about 10,000×g to sediment saidstructures in said solution; removing said structures from saidsolution; and mixing said structures in a second solution tosubstantially disperse said structures in said second solution, whereinsaid second solution is substantially free of said dispersal agent priorto mixing with said structures.
 20. A method of purifying single walledcarbon nanotube structures embedded within raw material, the methodcomprising: mixing said structures in a solution including an effectiveamount of a dispersal agent to to substantially separate said structuresfrom contaminants in said raw material.
 21. A single-walled carbonnanotube product comprising a solution including single-walled carbonnanotube structures coated with a dispersal agent, wherein saidstructures are substantially dispersed within said solution.
 22. Theproduct of claim 21, wherein said structures are further substantiallyfree of contaminants.
 23. The product of claim 21, wherein saiddispersal agent is selected from the group consisting of detergents withhigh surfactant activities, poloxamers, saponin glycosides,deoxycholates, cyclodetrins, chaotropic salts and ion pairing agents.24. The product of claim 21, wherein said dispersal agent comprises adetergent in said solution in an amount no greater than about 95% of acritical micelle concentration of said detergent.
 25. The product ofclaim 21, wherein said dispersal agent comprises a detergent in saidsolution in an amount at least about 50% of a critical micelleconcentration of said detergent.
 26. The product of claim 21, whereinsaid dispersal agent comprises a detergent having ahydrophilic-lipophilic balance value no greater than about 13.2.
 27. Theproduct of claim 21, wherein said dispersal agent is a detergentselected from the group consisting of Nonidet P-40, Poloxamer 188,polyoxyethylene sorbitol esters, ammonium bromides and ammoniumchlorides.
 28. The product of claim 21, wherein said dispersal agent isselected from the group consisting of cyclodextrins, saponin andtaurocholic acid.
 29. The product of claim 28, wherein said dispersalagent is in said solution in an amount no greater than about 500 mg/ml.30. The product of claim 28, wherein said dispersal agent is in saidsolution in an amount at least about 5 mg/ml.
 31. The product of claim21, wherein said dispersal agent is a cyclodextrin derivative selectedfrom the group consisting of methyl-β-cyclodextrin and2-hydroxypropyl-β-cyclodextrin.
 32. The product of claim 21, whereinsaid dispersal agent is a chaotropic salt selected from the groupconsisting of urea and guanidine.
 33. The product of claim 32, whereinsaid chaotropic salt is in said solution in an amount no greater thanabout 9M.
 34. The product of claim 32, wherein said chaotropic salt isin said solution in an amount at least about 6M.
 35. The product ofclaim 21, wherein said dispersal agent is a sulfonic acid, and saidsulfonic acid is in said solution in an amount no greater than about 100mM.
 36. The product of claim 35, wherein said sulfonic acid is selectedfrom the group consisting of 1-heptane-sulfonic acid and1-octane-sulfonic acid.